Cellulose sulfate esters are known materials which may be used as thickening agents in an aqueous medium. Preferred cellulose sulfate esters, as disclosed in my U.S. Pat. No. 4,138,535, and in the Reissue Application therefore, Ser. No. 033,455 filed Apr. 26, 1979, may be formed by sulfating a cellulose nitrite intermediate. Such cellulose sulfate esters have a generally more uniform distribution of sulfate ester groups among the polymer units of the cellulose. Accordingly, these cellulose sulfate esters have physical properties which are generally superior to cellulose sulfate esters prepared by other procedures.
As disclosed in my prior U.S. Pat. No. 4,138,535, it is advantageous in forming a cellulose nitrite ester that the cellulose reactant be in an activated state. Activation of the cellulose may be accomplished by treating the cellulose with water such that the cellulose contains about 4 to about 12 percent by weight of water uniformly distributed throughout the cellulose. An alternative way of preparing the activated cellulose is to treat cellulose containing about 4 percent by weight or more of water uniformly distributed throughout the cellulose with a highly polar aprotic solvent so as to reduce the water content of the cellulose. The temperature of such treatment is not critical. However, it should be kept in mind that enough time should be provided for the exchange of the water and the highly polar aprotic solvent and that the time required for the exchange is increased as the temperature is reduced. When formed in this manner, the activated cellulose may contain less than about 4 percent by weight of water. It is also desirable that the cellulose reactant be contacted with an aprotic solvent, such as dimethylformamide, prior to contact of the cellulose with the nitrosating reagent to reduce degradation of the cellulose and undesired side reactions.
When activated cellulose, as described, is nitrosated through treatment with dinitrogen tetroxide or nitrosyl chloride, as described in my U.S. Pat. No. 4,138,535, the resulting cellulose nitrite esters are more uniform than is the case when the cellulose nitrite esters are prepared by nitrosating dried cellulose. On sulfation of the cellulose nitrite esters prepared from activated cellulose, as described, the more uniform distribution of nitrite ester groups in the cellulose nitrite intermediate produces an even greater uniformity of sulfate ester distribution in the resulting cellulose sulfate ester product.
In the use of cellulose sulfate esters in forming thickened aqueous compositions, it would be desirable if the properties of the aqueous composition could be further enhanced by cross-linking the cellulose sulfate esters. This would, for example, permit the use of lower moleculor weight cellulose sulfate esters to produce a viscosity as high as, or higher than, that of high molecular weight cellulose sulfate ester. Also, if the cellulose sulfate ester could be cross-linked so as to improve the viscosity characteristics of aqueous compositions containing the cellulose sulfate ester, this would permit economies in the usage of cellulose sulfate esters. For example, by cross-linking the cellulose sulfate ester with a less expensive chemical, thickened aqueous compositions could be obtained which contain a smaller quantity of the cellulose sulfate ester than that previously required to produce the same viscosity.
Also, if cellulose sulfate esters could be cross-linked, this would permit the use of cellulose sulfate esters in the formation of cohesive, gel-like compositions. In this manner, for example, cellulose sulfate esters could be used for new applications in producing gelled products, such as gelled explosives, gelled cleaning compositions, gelled cosmetics, and the like. By providing aqueous compositions that are highly pseudoplastic and have a high yield point, cross-linked cellulose sulfate esters could then be employed in use applications in which product homogeneity is provided by a matrix structure such that suspended solids, for example, do not settle out but remain uniformly distributed throughout the gel.